Biodegradable plastics are of increasing industrial interest as replacements or supplements for non-biodegradable plastics in a wide range of applications. One class of biodegradable polymers is polyhydroxyalkanoates (PHAs). PHAs are biopolymers that microorganisms produce as temporary intracellular granules for intermediate storage of energy and carbon. PHAs recovered and purified from biomass exhibit diverse physicochemical properties that can make them suitable for use as ingredients in thermoplastics and elastomers. Compounds of plastics made with these polymers can be biodegraded by soil microbes and so they can be environmentally benign in the product or service as renewable resources and in the material fate in product end-of-life. There has therefore been a great deal of interest in establishing commercial applications for these polymers, particularly for a wide range of applications where biodegradation is integral to the product in service or is critical in end of product life handling.
Currently, the production of PHA relies on fermentation by wild type or genetically engineered pure cultures of bacteria or plants. The current process for manufacture of PHA polymer from microbes typically involves a biomass production stage, and an accumulation stage in which PHA becomes stored by the biomass to significant levels. Such production methods generally have high production costs, thus limiting widespread commercialization of PHA-based plastic compounds for conversion into engineering applications. Current research and practical developments have also been focusing on the use of open mixed cultures in order to lower the polymer production costs. These polymers may be produced using mixed-cultures of microbial biomass fed with waste residuals. This means that a PHA-rich biomass may be produced as an outcome of services in industrial and municipal effluent water quality management activities.
Mixed-cultures of microbial biomass can be enriched through selective environmental pressures in a biological water treatment process to contain a higher proportion of populations of species that can accumulate PHA. PHA is accumulated when such a biomass is stimulated into respiration with supply of readily biodegradable organic matter after having been subjected to a previous period of starvation. The period of starvation is without any supply of such readily biodegradable organic substrate.
Volatile fatty acids represent one commonly used substrate type to produce a PHA in a mixed culture biomass. When mixed cultures of biomass are fed with volatile fatty acids in a controlled way, they will typically accumulate Poly(3-hydroxybutyrate) (PHB), poly(3-hydroxyvalerate) (PHV), and/or copolymers of [Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)] (PHBV). PHB, PHV and PHBV are specific types of PHAs where the specific type and monomer distribution in the PHA is dependent on the type of substrate supplied to the biomass and the methods of the PHA accumulation. The specific type of PHA in a biomass may influence the conditions and requirements, such as the temperatures and kinetics necessary, for the polymer recovery from the biomass.
Recovery of PHA from biomass can be achieved by solvent extraction. PHA extraction from a biomass may or may not require a cell lysing/pre-treatment process, but will typically require the use of an organic solvent as a means to selectively dissolve and separate the polyester from non-dissolved non-PHA cell residue.
The dissolved polymer solubility, once separated from biomass residue, in a PHA rich solvent may be reduced with temperature change, distillation, and/or addition of a more polar co-solvent. Thus a polymer residual can ultimately be separated from the extraction solvent and the polymer may also be further washed with solvents to produce an even purer polymer resin.
Although PHAs may be processed in conventional melt processing equipment, such as extrusion and film blowing, challenges have been encountered. Challenges may be in the material melt processing and/or in the material properties once the material has been formed into a commercial product. For example, PHA containing high levels of 3-hydroxybutyrate monomer may be limited in scope of product application due to brittleness. In some cases, such products of PHA based plastics may not exhibit an acceptable degree of toughness for the intended application. Thus, a need exists for influencing toughness, for example, as well as other mechanical properties of PHA-based plastics.